Primary standard
= Primary standard
{wiki}
A <primary standard> supplies a reliably known reacting inventory from which a <titrant> <concentration> can be determined.
The defining advantage is known high purity: the weighed portion can be assigned a known <amount of substance>. A <titrant> whose <concentration> is established against that inventory is a secondary standard solution in IUPAC terminology.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry/standard solution>[IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry; Gold Book, standard solution, S05924; definitions of primary standard and secondary standard solution; underlying source Orange Book, second edition, p. 48]{ref}
An accurate balance determines total <mass>, including any unwanted <water> or other material. To infer reacting inventory, one also needs known composition, stability during storage and weighing, complete transfer and dissolution, and a sufficiently complete <chemical reaction> of known <stoichiometry>. A finely resolved balance cannot establish these chemical facts. <Potassium hydrogen phthalate> is useful because a suitably prepared portion provides a known acid inventory for <sodium hydroxide> <titrant standardization>; the <KHP standardization protocol> below shows how that claim is put into practice.
Preparation belongs to the particular material and method. For example, the MIT handout specifies drying <potassium hydrogen phthalate> at $110\,{}^\circ\mathrm C$ for $1.5$ hours or overnight, cooling in a desiccator, and weighing by difference; its footnote identifies removal of superficial moisture as the purpose. This is a condition of that <potassium hydrogen phthalate> procedure, not a universal instruction to heat every <primary standard> in the same way.<The Potentiometric Titration of an Acid Mixture (MIT 5.310 laboratory handout)>[Printed Titration-5, §IV.2 and footnote 5: drying, cooling, weighing by difference, and superficial moisture]{ref}